Planar waveguides are cost-effective devices to provide lighting for e.g. liquid crystal displays or keysets. Light which is initially provided e.g. by an external light emitting diode (LED) may be distributed to a larger area by means of a planar waveguide. The use of thin planar waveguides may facilitate reducing size, weight and manufacturing costs of a portable device.
Referring to FIG. 1, optical power may be lost when a broad light beam B1 emitted by a light source 200 impinges on the edge of a thin planar waveguide 10, as the beam B1 overlaps only partially with the edge.
Referring to FIG. 2, optical power may be also lost when the light source is misaligned with respect to the edge of a thin planar waveguide 10. The coupling efficiency may be degraded due to misalignment also in a case when the vertical dimension of the light beam is equal to or smaller than the thickness of the planar waveguide. As the thickness of the planar waveguide may be e.g. 0.2 mm, and the manufacturing tolerances may be e.g. 0.5 mm, the alignment problem may be significant in mass production.
The arrangements of FIGS. 1 and 2 are herein called “directly edge coupled” arrangements.
FIG. 3 of a patent publication US2005/0259939 discloses a further directly edge coupled arrangement wherein a planar waveguide has a tapered edge portion to facilitate coupling of light into the planar waveguide.